Lighting or surge arresters are typically connected to power lines to carry electrical surge currents to ground, thereby preventing damage to lines and equipment connected to the arresters. Arresters offer high resistance to normal voltage across power lines, but offer very low resistance to surge currents produced by sudden high voltage conditions caused by, for example, lighting strikes, switching surge currents or temporary overvoltages. After the surge, the voltage drops and the arrester normally returns to a high resistance state. However, upon arrester malfunction or failure, the high resistance state is not resumed, and the arrester continues to provide an electrical path from the power line to ground. Ultimately, the line will fail due to a short circuit condition or breakdown of the distribution transformers, and the arrester will require replacement.
To avoid line lockout, disconnector assemblies are commonly used in conjunction with arresters to separate a malfunctioning arrester from the circuit and to provide a visual indication of arrester failure. Conventional disconnector assemblies have an explosive charge to destroy the circuit path and physically separate the electrical terminals. Examples of such disconnector assemblies are disclosed in U.S. Pat. No. 5,952,910 to Krause and U.S. Pat. Nos. 5,057,810 and 5,113,167 to Raudabaugh, as well as U.S. Pat. No. 5,434,550 to Putt, U.S. Pat. No. 4,471,402 to Cunningham and U.S. Pat. No. 4,609,902 to Lenk, the subject matter of each of which are hereby incorporated by reference.
Traditionally, polymer-housed distribution class arresters are assembled with a ground end insulating bracket that physically supports the arrester, as well as isolating the ground end of the arrester from the system ground in the event of arrester service failure. A ground lead connector, or isolator, connects the ground end of the isolator to the system neutral or ground wire.
In normal service conditions, the arrester grading current flows through the ground lead isolator. If the arrester fails, the arrester 60 Hz fault current flows through the failed arrester and through the ground lead disconnector, which causes the ground lead disconnector to operate. The disconnector disconnects from ground, thereby effectively isolating the failed arrester from ground. Separating the arrester from ground allows the utility to provide uninterrupted service to its customers. This also facilitates identifying the failed arrester so that it may be replaced with a new arrester.
Existing disconnectors typically have a grading component in parallel with a sparkgap. The grading component and sparkgap are located close to a detonating device, such as an unprimed cartridge. The grading component conducts the arrester grading current under normal service conditions. If arrester failure occurs, the arrester grading current increases from a few milliamperes to amperes or thousands of amperes, depending on the utility system grounding at the arrester location. This high current flow causes voltage to develop across the disconnector grading component. When voltage reaches a predetermined level, the parallel sparkgap sparks over, thereby causing heat build-up on the cartridge. The cartridge then detonates and separates the ground lead connection.
Typically, the grading component is a low voltage precision resistor, a high power resistor, or a semi-conductive polymer material. However, these grading components tend to fail during prolonged temporary overvoltage situations. Failure of the grading components can prevent disconnectors from properly detonating. A need exists for a disconnector providing a more reliable cartridge detonation.
Furthermore, existing grading components are often significantly damaged during durability testing, which results in deterioration of the electrical integrity of the disconnector. A deteriorated grading component may result in a degraded time-current deterioration characteristic. A need exists for a grading component that is not significantly deteriorated by durability testing.
A need exists for an improved disconnector assembly for an arrester.